Exemplary embodiments of the invention relate to getters for sorbing gasses, and in particular to composite material getters. Other exemplary embodiments relate to methods for production of same.
In many technologically advanced applications gas sorption is achieved with non-evaporable getter (NEG) materials. NEG materials are frequently found in two types of applications. In a first type of application a NEG material is used to purify a gas stream by sorbing unwanted species. For example, in the semiconductor industry bothersome species such as hydrogen, oxygen, nitrogen, water, oxides of carbon, and oxides of nitrogen can be removed from noble gas streams utilizing NEG materials. Similarly, gasses used in the manufacture of certain gas-filled devices such as light bulbs are purified to provide advantages such as improve filament lifetimes.
In a second type of application a NEG material is used to maintain a high degree of vacuum within a sealed enclosure. Processing chambers are common examples of such enclosures in the semiconductor industry. Such enclosures can also be found, for example, in thermal insulation devices such as thermal bottles, dewars, microelectronic packages, and insulated pipes for oil extraction and for oil transport in arctic and undersea regions. Sealed enclosures for these applications typically include an inner wall and an outer wall with an evacuated volume maintained between the two walls. For oil extraction and transport it is frequently necessary to use insulated pipes in order to prevent excessive cooling of the fluid. Such cooling can cause the heavier components of the oil to solidify with a resulting increase in the total viscosity thereof, potentially creating a blockage.
NEG materials may include metals such as zirconium and titanium and alloys based on these metals and compounds thereof. Such alloys can include one or more other elements selected, for example, from among the transition metals and aluminum, and their oxides. NEG materials have been the subject of several patents. U.S. Pat. No. 3,203,901 describes Zr—Al alloys, and particularly an alloy whose weight percent composition is Zr 84%-Al 16%, produced and sold by SAES Getters S.p.A., Lainate, Italy, under the name St 101®. U.S. Pat. No. 4,071,335 describes Zr—Ni alloys, and particularly an alloy whose weight composition is Zr 75.7%-Ni 24.3%, produced and sold by SAES Getters S.p.A., Lainate, Italy, under the name St 199®. U.S. Pat. No. 4,306,887 describes Zr—Fe alloys, and particularly an alloy whose weight composition is Zr 76.6%-Fe 23.4%, produced and sold by SAES Getters S.p.A., Lainate, Italy, under the name St 198®. U.S. Pat. No. 4,312,669 describes Zr—V—Fe alloys, and particularly an alloy whose weight percent composition is Zr 70%-V 24.6%-Fe 5.4%, produced and sold under the name St 707®. U.S. Pat. No. 4,668,424 describes Zr—Ni-A-M alloys, where A represents one or more rare earth elements, and M represents one or more elements selected from among cobalt, copper, iron, aluminum, tin, titanium, and silicon. U.S. Pat. No. 5,961,750 describes Zr—Co-A alloys, where A is an element selected from among yttrium, lanthanum, the rare earth elements, and mixtures thereof. This patent application particularly discloses an alloy whose weight percent composition is Zr 80.8%-Co 14.2%-A 5%, produced and sold by SAES Getters S.p.A., Lainate, Italy, under the name St 787®.
The sorption of gasses by NEG materials appears to occur in two stages. A first stage is the superficial chemisorption of the gaseous species onto the surface of the NEG material, generally accompanied by the dissociation of the species into its constituent atoms. In a second stage the constituent atoms diffuse into the bulk of the NEG material. In the case of hydrogen sorption, as hydrogen atoms spread inside the material they first form solid solutions, even at low temperatures. As the hydrogen concentration increases, hydrides such as ZrH2 are formed. The sorption capacity for hydrogen is high even at low temperatures.
The second stage is different for elements such as oxygen, carbon, and nitrogen. At relatively low temperatures (generally lower than about 300-500° C., according to the type of the NEG material) only superficial chemisorption occurs and surface layers of oxide, carbide or nitride compounds are formed. These layers effectively block bulk diffusion from occurring. At higher temperatures the oxygen, nitrogen, and carbon atoms are able to diffuse into the NEG material, thus regenerating a clean surface for further gas sorption. Therefore, surface cleaning can be achieved continuously by constantly maintaining a NEG material at a sufficiently high temperature. Alternately, the surface of a NEG material maintained at a low temperature can be cleaned by periodically bringing it to a sufficiently high temperature. This latter process is commonly known as an “activation” treatment, and may be carried out at regular intervals or when a loss of sorption capacity is observed.
However, there are many applications for NEG materials where they cannot be heated to create an activation of the materials. Such applications include maintaining high vacuum levels in sealed enclosures like those found in X-ray tubes, field-emission flat panel displays, thermal bottles, dewars, fluorescent lamps, and the insulated pipes used in oil extraction and transport. Another important application of this kind is in batteries, both of the rechargeable kind such as Ni-metal hydride batteries, and of the non rechargeable kind, such as conventional alkaline batteries. As is well known in the art, batteries include an anode, a cathode, and an electrolyte disposed between them, all contained within a casing. Both alkaline and rechargeable batteries, under certain operating conditions, may release hydrogen causing the casing to swell and creating a risk of explosion. The buildup of hydrogen may also occur in air-tight containers, such as munitions and pyrotechnics containers. Due to the presence of hydrogen, it may be extremely dangerous to activate the NEG materials by heat.
In these low-temperature applications the sorption of relatively small quantities of oxygen, nitrogen or carbon produces a passivation layer on the surface of the NEG material, as previously described, which prevents further gas sorption and reduces sorption capacity of the NEG material to a fraction of its theoretical value. Further, the passivation layer blocks hydrogen sorption which, as already explained, would otherwise occur to a high extent even at room temperature.
In some applications that employ NEG materials the presence of hydrogen can be especially harmful. In the case of thermal insulation applications, this is because hydrogen is an excellent thermal conductor. Therefore, hydrogen in an evacuated volume, even in small quantities, notably worsens the thermal insulating property thereof. The presence of hydrogen in the gaseous filling mixture of lamps modifies the discharge conditions, and thus both prevents the lamp from functioning optimally and generally shortens its life. In addition, hydrogen is dangerous because it oxidizes rapidly if exposed to a spark, resulting in an explosion. This is of particular concern when the hydrogen outgases from components that are themselves explosive or inflammable.
The formation of a getter deposit may be accomplished according to several techniques including, by way of example but not limitation, sputtering, evaporation, and deposition onto a support. The sputtering technique deposits films with a thickness from fractions of micrometers (micron, μm) up to some tens of microns. Such films typically have an excellent adhesion to a substrate and are resistant to the loss of particles. With sputtering it is also possible to control (at least within certain limits) the morphology of the deposit, optimizing the same for the expected application. For example, a columnar morphology may show a high specific surface area (surface area per unit of weight of the deposit). Moreover, with this technique it is possible to control with a high precision level even the lateral location of the deposit, ensuring the getter deposit is properly aligned. Due to the advantages of sputtering, it is a preferred technique in many applications. U.S. Pat. Pub. 2004/0253476 describes formation of a multilayer getter structure.
Getter materials may be pressed and sintered in order to form pellets, disks, or other useful shapes. Powders of material may also be deposited onto a generally planar substrate using techniques such as cold rolling or screen-printing. Formation of pellets, the use of containers, and cold lamination are well known in the field of powder metallurgy, and the details of the screen-printing technique as applied to getter materials are described in U.S. Pat. No. 5,882,727.
Getter material is typically treated to clean the surface of the getter material, which can activate the getter material. A getter material, even if it has already been chemically activated, may undergo a further thermal activation. At elevated temperatures, components of a device incorporating the getter tend to release gasses. The activation is usually carried in a chamber that is pumped down to remove these gasses, but in the final stages the getter is enclosed within the device. When the cavity is closed, pumping from outside becomes ineffective. The environment within the device is then controlled by pumping in inert or non-reactive gasses or maintaining a vacuum. From that point, the getter absorbs outgassing from components into the environment.
Activation of getter materials may be inconvenient in certain situations, such as maintenance. Since getters are typically heat-activated, the replacement of getters may be a time-consuming task that involves heating the getter material to a requisite temperature before deploying it. In practically every case, a getter material that does not require activation is more convenient than one that does.
Since getters are often relatively large and bulky, there are ongoing efforts to reduce their size. Still, it is difficult to place some getters into certain environments. For instance, locating getter materials inside or around dewars on, for example, MRI machines may be impossible without retrofitting or redesigning the machines. In some cases, available getters are simply too thick or cannot be spread over an area effectively. Much of the bulk may be from the substrate, which is often ceramic.
Many getter materials particulate over time. It may be undesirable to have pieces of getter material fall off into an environment for functional or even aesthetic reasons. For example, a relatively unsophisticated viewer may feel that a device with a particulating getter is broken or of low quality. Particles of spent material could impact the mechanical or electronic functionality of a device.
Many getters produce water vapour or outgas organics. This may be undesirable in certain situations. Some corrosive organics, such as chlorine and fluorine, are outgassed, potentially causing damage.
Getter materials can be expensive. The cost may be due to an expensive substrate material, and/or the getter material itself. Furthermore, it is typically impossible to cost-effectively recycle materials of the getter because of characteristics of the substrate, such as the relative cost of the substrate compared to the getter material, and the difficulty of removing the getter from the substrate. Since some getters make use of relatively expensive materials, such as palladium, this can be wasteful.
Getter materials can be difficult to transport. This is due to reasons that include the fragility of the getter material, the fragility of the substrates, the reactivity of the getter material, or the bulkiness of the getter or the substrate to which the getter is affixed.
Getter materials may be fragile because they are, for example, brittle, breakable, or susceptible to degradation through environmental or other contamination. After a getter is employed, it may be fragile as well. Such fragility often includes susceptibility to contamination such as dirt, temperature, light, moisture, and other environmental factors.
Getter materials can also be difficult or time-consuming to produce. For example, getter substrates must typically be boiled in multiple solvents before having getter material applied. Getter materials often must be soldered into place to keep them from moving around. Other getter materials are heavy and difficult to deposit where desired.